At 18/19 mph you must be at Speed Code 4 now if those are stock height tires. The next best speed you can get is an Alltrax XCT300 controller upgrade
https://www.revolutiongolfcars.com/p...roller-300-amp. This is a Bolt-On replacement to your stock Curtis controller and will yield 23-24 mph with stock tires and a good pack. More speed with taller tires.
Not sure about your DOT question.